The proportion of erythro form in β-O-4 structures was higher in the tension wood than in the opposite wood of the stem of yellow poplar (Liriodendron tulipifera). The ratio of erythro form and the methoxyl group content in the 7 positions in the stem were correlated (correlation coefficient R =0.98), suggesting that the type of aromatic ring, syringyl or guaiacyl, is one of the factors which stereochemically controls the ratio of erythro and threo forms of β-O-4 structures during lignin formation. Investigation for more than 20 species of woods confirmed the relationships between stereo structure of lignin side-chain and aromatic structure. Structural change of lignin during milling of wood was analyzed. Comparison of results obtained by nitrobenzene oxidation and ozonation analysis suggested that structural changes of lignin had mainly occurred on side-chain part without the participation of aromatic part. Ozonation method was also applied to the analysis of structural change of lignin during acid or alkali treatments and provided numerous results which was never obtained by other methods.